1. Field of the Invention
The invention relates generally to system and method for controlling thickness of parison walls for use with plastic molding machines. More specifically, the invention relates to a parison thickness control system and a method for use with parison extruders in plastic molding machines, which can control the wall thickness of the extruded parison as desired, and further control the operation timings of associated devices in synchronism with an extrusion or forming cycle of the parison.
2. Description of the Background Art
As is known, plastic is melted in the parison extruder and is forced through an annular opening defined between an extruder die and a mandrel in the form of a tube so as to be formed as a parison. Thickness of the parison walls is controlled according to a configuration and thickness of a finished product.
Japanese Patent First (Unexamined) Publication (Tokkaisho) No. 63-82707 discloses one of the conventional parison thickness control systems. As shown in FIG. 1, a desired parison wall thickness A is defined over the entire length L of the parison in the form of a broken line, which data is stored in the memory unit. The system derives a target parison wall thickness A for a corresponding parison length per unit time during the parison extrusion or forming cycle based on the stored data using the method of interpolation, and adjusts an annular gap between the extruder die and a core of the mandrel according to a value of the derived target parison wall thickness A. The system sequentially derives a parison length which is to be extruded during the unit time based on the derived target parison wall thickness A and an extrusion amount of the melted plastic. The system further derives the accumulated parison length sequentially for deriving the corresponding target parison thickness A using the above-noted stored data and for detecting the termination of the parison extrusion cycle by comparing it with the stored entire parison length L. The termination of the parison extrusion cycle may also be detected by comparing an accumulated elapsed time with the stored entire parison extrusion cycle time.
In the foregoing background art, however, since variations in the target parison wall thickness are defined by the broken line, i.e. in the form of an angular pattern, when discontinuous points influence the finished product, ring lines are likely to be formed on the outer surface of the finished product at portions corresponding to angled portions of the broken line in FIG. 1, or unexpected uneveness of the thickness is likely to be generated in the finished product at the angled portions of the broken line, to weaken the structure of the finished product. In order to avoid this, it is necessary to make the wall thickness of the finished product unnecessarily thicker as a whole, which is disadvantageous in view of the cost.
Further, in the foregoing background art, the parison thickness control system can not control operations of other associated devices in the plastic molding machine in timed synchronism with the parison extrusion cycle. Specifically, in case of the continuous extrusion of the parison, the timed synchronism should be necessary between the parison extrusion cycle and the entire operation cycle of the plastic molding machine including, for example, the closing and opening of a mold having two mold halves, the cutting of the parison by a parison cutter and the transfer of the mold to another station.